Process for producing hard elements of wood

ABSTRACT

PCT No. PCT/SE94/01098 Sec. 371 Date May 14, 1996 Sec. 102(e) Date May 14, 1996 PCT Filed Nov. 18, 1994 PCT Pub. No. WO95/13908 PCT Pub. Date May 26, 1995A method of producing hard wooden elements by compressing wooden blanks at an isostatic pressure greater than 800 bars, preferably greater than 1000 bars.

FIELD OF THE INVENTION

The present invention relates to a method of producing hard woodenelements, and then in particular sawn wooden elements.

BACKGROUND OF THE INVENTION

It is known to produce hard wooden elements, such as sheets of floorboarding, by compressing different types of wood products inconventional presses. DE 0 601 162 describes one example of a woodpressing technique. Wood sheets of limited size layered withsteam-heated metal plates are stacked in a steam operated press. Apiston driven by pressurized steam functions to press vertically on thestack of metal plates/wood sheets from beneath the stack. Side platesare located on two of four sides, therewith enabling the wood to expandin two directions as it is compressed. Because of this possibility forthe wood to expand, there is a limit on the maximum pressure to whichthe wood can be subjected. Deformation of the wooden sheet becomes verypronouned when the wood is subjected to high pressure, and there is alsoa danger that the wooden sheet will be forced out of the press. It isnot therefore possible to produce hard wooden elements under very highpressures with the technique taught by DE 0 601 162.

U.S. Pat. No. 3,621,897 describes a method of pressing wooden pieces oflimited size in a press mould so as to obtain a patterned surface. Thewood is pretreated by immersing the wood in a water/pyridine mixture forsome minutes. After being dried, the wood is pressed under hotconditions (at about 180° C.) in a metal mould that will produce thedesired pattern. Nothing is mentioned as to which pressure shall beapplied. According to this patent specification it is well known thatthe plasticity and compressibility of natural wood is very low. Thepyridine treatment makes the wood soft and pliable, therewith enablingthe wood to be pressed in the mould without the wood cracking. However,the pyridine impregnation process constitutes an additional treatmentstage which complicates the manufacturing process. Furthermore, pyridineis a skin irritant and is extremely toxic. This technique does not allowwood to be compressed without prior impregnation of the wood withpyridine, if the wood is not to crack. This is a serious drawback, inview of the toxicity of pyridine. Furthermore, it is evident that thewood is pronouncedly deformed when practicing this technique. As shownin the figures, the wood is flattened pronouncedly when compressed.Neither does this technique enable hard wooden elements to be obtainedwhile essentially retaining the shape of the wood after compression,since the technique is based on the wood being pressed in a mould, andmoulds, after all, are not so flexible as to provide a perfect fit witheach piece of wood.

U.S. Pat. No. 2,666,463 describes a wood pressing technique in which thewood is first heated quickly so as to reduce its moisture content toabout 15% and to render the lignin plastic, whereafter the heated woodis pressed in a mould to reduce its volume instead of obtainingflattening of the wood at a constant volume. According to this patentspecification, the wood is compressed at high pressures 800-2000 psi(55-138 bars). When pressures of these high magnitudes are applied fromone direction, the material is subjected to high stresses and strainsand in order to obtain an acceptable result it is necessary for the woodstarting material to be even and relatively homogenous. Since knots aregenerally much harder than the remainder of the wood, compression of aknot-rich piece of wood is liable to give rise to ugly crack formationsand may totally pulverize the knots. The pressed wooden element is alsodeformed to the shape of the mould used. The technique taught by U.S.Pat. No. 2,666,463 cannot therefore be applied to compress knot-richwooden elements or wooden elements which are inhomogeneous in otherways, or to compress wooden elements of any chosen and/or irregularshapes with acceptable results.

The press devices described in the aforesaid patent specificationsgenerate all of their pressure by pressing pistons against sheets which,in turn, distribute and forward the pressure to the blank to be pressed.No homogenous pressure load is obtained with presses of this kind, andthe highest pressure on the blank is located in the centre of the sheetin a region opposite the region at which the pressing piston is attachedto the sheet. The pressure then decreases further out in the peripheralregion of the sheet. It is thus not possible to generate high homogenouspressures over large surface areas with the aid of the aforesaid typesof press.

There is therefore a need for a method of producing hard wooden elementsby packing the elements in a press where the wood will not be deformedbut will essentially retain its shape although its volume will bedecreased, where no toxic or otherwise unpleasant impregnation chemicalsneed be used, and where starting materials which contain knotsinhomogeneities or irregularities will not have an impairing effect onthe result or cause any significant change in shape apart from saidreduction in volume. There is also a need to be able to apply highpressure forces to large surface areas in the manufacture of table tops,table leaves or flooring materials.

The present invention eliminates the aforesaid deficiencies of knowntechniques in an unexpected and advantageous manner.

SUMMARY OF THE INVENTION

The present invention relates to a method in which a wooden blank iscompressed in a press which is capable of generating a high isostaticpressure, preferably a pressure greater than 800 bars, and even morepreferably greater than 1000 bars.

The term "wooden blank" as used here denotes different types of woodengoods, such as sawn timber, particle board, chipboard, wallboard,plywood sheets, and so on.

The invention is particularly useful in connection with the processingof wood waste and surplus wood.

By isostatic pressure is meant a pressure which is equally as large inall directions in space. The pressure at an arbitrary point within aliquid or a gas mass is an example of isostatic pressure in nature.Thus, a press which generates an isostatic pressure is able to exertequally as large forces in all directions and at all points. Thisenables a homogenous wooden blank to be compressed with regard to volumewithout changing the shape of the blank. An isostatic press whichoperates at high pressures is able to exert the same high pressureacross the whole of the outer surface of an object and not only on asmall surface area thereof, as is the case with conventional presses.This enables extremely high pressures to be applied without destroyingthe blanks.

SE-C-452 436 describes a press of the pressure cell type. The press isused primarily within the aircraft and automobile industry formanufacturing difficultly shaped sheet metal elements in small serieswith the aid of a compression moulding process. A piece of sheet metalis placed on a hard substrate (tool) which has a relief image thatcorresponds to the desired appearance of the finished sheet metal pieceand whose configuration is not changed by the pressure. A membrane, forinstance, a rubber membrane, is mounted on the sheet-metal workpiece.The pressure is then generated, by pumping pressurized hydraulic fluidbehind the membrane, so as to transfer the substrate image onto thesheet-metal workpiece.

It has now been found that a press of the kind described in SE-C-452 436can be used in a manner which causes the press to exert an isostaticpressure on a blank. When the hard substrate or tool mentioned above isreplaced with a tray which is either covered with or filled with piecesof plastic or elastomeric material, for instance rubber or elasticpolyurethane, which when subjected to pressure will conform to the shapeof the blank instead of shaping the blank, there is obtained a state inwhich the blank is subjected to isostatic pressure. The working fluidbehind the membrane exerts the same pressure in all directions andbecause the membrane and the substrate both change their shape andconform to the blank, those pressure forces that act from outsidedirectly on the blank will also be equally as large in all directions.

In spite of the pressure being isostatic, it is possible that the woodenblanks will be deformed slightly when subjected to pressure and becomeslightly narrower on that side thereof which lies proximal to the pressmembrane. This is because the friction against the plastic/elasticmaterial in the tray counteracts shrinkage of the wooden blank locally.This can be alleviated partially by suitable selection of theplastic/elastic material used, and also by suitably positioning theblanks in the tray prior to applying pressure.

The inventive method cannot be applied satisfactorily to wooden blanksthat have been taken from newly worked timber or from other timber thathas an excessively high moisture content. Since the liquid present inthe tree is not compressible, there will be no reduction in the volumeof a pressed moist wooden blank. On the other hand, wooden blanks whichhave an excessively low moisture content will crack when subjected topressure. The compressibility of the wooden blanks is thus governed by amoisture content within a range which has a top limit valuecorresponding to the maximum moisture content that can be allowed inorder to obtain a desired reduction in volume, and a bottom limit valuewhich corresponds to the highest moisture content at which the woodenblanks will begin to crack in conjunction with the pressing operation.This range varies between different types of wood and different woodqualities. The person skilled in this art, however, will be able toassess whether or not a batch of wooden blanks can be pressed, bypressing a sample blank taken from the batch in question.

Commercially available wooden blanks having normal moisture contents canbe pressed advantageously by means of the inventive method. Blanks ofthis kind that are subjected to extremely high pressure forces (above800 bars, particularly above 1000 bars) obtain in this way newadvantageous and unexpected properties. The volume of the wooden blankscan be reduced by half, without damaging the blanks end without changingtheir shapes to any appreciable extent, which must be consideredparticularly surprising, especially in view of the fact that in earliertechniques very low pressures have been applied in combination withimpregnating the wood, in order to avoid the formation ofpressure-generated cracks. The fact that the wooden blanks are notdamaged and their shape essentially retained can be seen by theintactness of the growth rings, although these rings are now closertogether. All wood that is accommodated in the aforesaid tray, ortrough, can be pressed almost irrespective of shape while essentiallyretaining the original shape of the blanks, with the exception of saidreduction in volume. When using a sufficiently large press arrangement,it is also possible to press a commercially acceptable surface, i.e. asurface preferably larger than 1 m².

No signs of pressure-caused crack formations have been found, despitepressing at temperatures which lie only slightly above room temperature(25°-60° C.) and despite not previously impregnating the wood with someplasticizing substance. Knots present in the wood are also compressedand remain intact.

As beforementioned, wood that has been pressed by means of the inventivemethod has clearly improved properties over the starting material. Thehigh pressure forces applied (higher than 800 bars, preferably higherthan 1000 bars) impart to treated pine wooden blanks a hardness anddurability comparable to that of oak, while softwood blanks, such asaspen blanks, obtain a hardness which enables the product to be used inthe furniture industry, in the manufacture of table tops, table leaves.The density of the treated wooden blanks increases of course, and oakthat has been pressed in accordance with the invention will sink inwater for instance. Because of its compressed structure, wood that hasbeen pressed in accordance with the invention will not ignite or burn asreadily as natural wood. Normally, only the outermost surfaces of woodtreated in accordance with the invention will be blackened when cominginto direct contact with fire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the actual press chamber.

FIG. 2 illustrates the pressing of wood in the same press as thatdescribed above, but in which the blanks 13 have been mutually stackedwith rubber scrap 16 disposed between each blank.

FIG. 3 illustrates the configuration of a wooden blank that has beenembedded in rubber scrap in the pressing operation.

FIG. 4 illustrates the configuration of a wooden blank which has notbeen embedded in rubber scrap in conjunction with the pressingoperation.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in more detail with reference tonon-limiting embodiments thereof.

The actual press is referenced 10 and includes a top part 11 and abottom part 12 which are mutually joined in a manner (not shown) whichenables the press to take-up very large pressure forces. The woodenblanks 13 have been placed on the bottom press part (the tray) 12.Rubber scrap 16 has been packed around the blanks.

The top press part includes a rubber membrane 17 which forms the bottomdefining surface of a chamber 18 and which is moved together with thepress part 11 down against the bottom press part at the beginning of apressing operation. The membrane 17 therewith extends across the rubberscrap 16 and the wooden blanks 13 and the outer parts of the membranelie against the bottom press part 12.

The chamber 18 contains a working fluid which subjects the wooden blanksto a corresponding isostatic pressure, by virtue of the membrane and therubber scrap laying between the membrane and the blanks transmittingpressure uniformly to all parts of the blanks.

EXAMPLE 1

A pinewood blank containing knots was pressed in accordance with theinventive method. The pressure cell press used was a QUINTUS-press (ABBPressure Systems AB, Vasteras, Sweden) which delivered a highestpressure of 1400 bars.

A part of a wooden blank was sawn off and saved for later comparison.The remainder of the wooden blanks 13 were placed on the bottom presspart (the tray) 11. Rubber scrap 16 was then packed around the blanks,to fill-out cavities in the press and so that pressure would betransmitted to all sides of the blanks.

The wooden blanks were then subjected to a pressure of 1030 bars at atemperature of 35° C. for a period of 1.5 minutes. The pressure was thenrelieved and the press parts separated whereafter the wooden blanks wereremoved from the press and compared with the non-pressed sample piece.The cross-section surfaces of the wooden blanks and the hardness thereofwere measured. The results of these measurements are set forth in thefollowing table. The pressed blanks had retained their shapes and thegrowth rings and knots were found to be intact. The wooden blanks weresubjected to a simple burning test in which it was established that thenon-pressed wood sample caught fire relatively easily, whereas thepressed blanks were only lightly blackened on their respective surfaces.

                  TABLE 1                                                         ______________________________________                                                   Cross-section  Hardness                                                      width (mm)                                                                              height (mm)                                                                             (Rockwell)                                      ______________________________________                                        Pressed blank                                                                             65          22        99                                          Untreated blank                                                                           77          33        86                                          ______________________________________                                    

EXAMPLE 2

Oak blanks were pressed in the same press as that used in Example 1. Theblanks were pressed in the same manner as that aforedescribed, althoughno rubber scrap was packed around the blanks, but that each blank wasplaced directly on an elastic rubber covering on the tray bottom. Thefollowing results were obtained:

                  TABLE 2                                                         ______________________________________                                        bottom width (mm)                                                                             top width (mm)    height (mm)                                 ______________________________________                                        Prior to                                                                             45           45          45  15                                        pressing                                                                      After  43           38              11                                        pressing                                                                      ______________________________________                                    

Because no rubber scrap was packed around the blanks, the upper cornerswere rounded, although the bottom sides of the blanks were not affectedappreciably by the pressing operation.

We claim:
 1. A method of producing hard wooden elements, whichcomprises: compressing one or more wooden blanks by applying to saidwooden blanks an isostatic pressure greater than 800 bars.
 2. A methodaccording to claim 1, wherein the isostatic pressure is greater than1,000 bars.
 3. A method according to claim 1, wherein the wooden blanksare sawn wood products.
 4. A method according to claim 1, wherein theisostatic pressure is applied for a total time period of less than 5minutes.
 5. A method according to claim 4, wherein the isostaticpressure is applied for a total time period of less than 3 minutes.
 6. Amethod according to claim 1, wherein the step of compressing is carriedout at a temperature of between 25° and 60° C.
 7. A method according toclaim 1, wherein the isostatic pressure is generated with the aid of apressure-cell press.
 8. A method of producing hardwood elements, whichcomprises:placing a plurality of wooden blanks in a pressure-cellisostatic press, and compressing said wooden blanks by applying to saidwooden blanks an isostatic pressure greater than 800 bars for a periodof less than 5 minutes and at a temperature between 25° and 60° C.